1. Field of the Invention (Technical Field)
The present invention relates to creation and use of long folded optical paths in a compact structure for use with lasers in making optical measurements or systems.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Multiple pass optical cells with dense spot patterns are useful for many applications, especially when the cell volume must be minimized relative to the optical path length. Present methods to achieve these dense patterns can use matched pairs of either astigmatic mirrors or cylindrical mirrors. In both cases, the optical beam exiting the cell needs to be separated from the incoming beam, which enters the cell through the center of the front mirror. This separation is difficult due to the small angle between the beams. This invention describes a new, simple approach to collect and measure the output beam intensity using a separate hole in the center of the rear mirror.
Multiple pass optical cells are used to achieve very long optical path lengths in a compact footprint and have been extensively used for absorption spectroscopy (White, J. U., “Long Optical Paths of Large Aperture,” J. Opt. Soc. Am., vol. 32, pp 285-288 (May 1942); Altmann, J. R. et al., “Two-mirror multipass absorption cell,” Appl. Opt., vol. 20, No. 6, pp 995-999 (15 Mar. 1981)), laser delay lines (Herriott, D. R., et al., “Folded Optical Delay Lines,” Appl. Opt., vol. 4, No. 8, pp 883-889 (August 1965)), Raman gain cells (Trutna, W. R., et al., “Multiple-pass Raman gain cell,” Appl. Opt., vol. 19, No. 2, pp 301-312 (15 Jan. 1980)), interferometers (Herriott, D. H., et al., “Off-Axis Paths in Spherical Mirror Interferometers,” Appl. Opt., vol. 3, No. 4, pp 523-526 (April 1964)) and other resonators (Yariv, A., “The Propagation of Rays and Spherical Waves,” from Introduction to Optical Electronics, Holt, Reinhart, and Winston, Inc., New York (1971), Chap. 2, pp 18-29).
These cells have taken the form of White cells (White, J. U., “Long Optical Paths of Large Aperture,” J. Opt. Soc. Am., vol. 32, pp 285-288 (May 1942)), integrating spheres (Abdullin, R. M. et al., “Use of an integrating sphere as a multiple pass optical cell,” Sov. J. Opt. Technol., vol. 55, No. 3, pp 139-141 (March 1988)), and stable resonator cavities (Yariv, A., “The Propagation of Rays and Spherical Waves,” from Introduction to Optical Electronics, Holt, Reinhart, and Winston, Inc., New York (1971)).
The stable resonator is typified by the design of Herriott (Herriott, D. H., et al., “Off-Axis Paths in Spherical Mirror Interferometers,” Appl. Opt., vol. 3, No. 4, pp 523-526 (April 1964)). The simplest such Herriott cell consists of two spherical mirrors of equal focal lengths separated by a distance d less than or equal to four times the focal lengths f of the mirrors. This corresponds to stable resonator conditions. A collimated or focused laser beam is injected through the center of a hole in one of the mirrors, typically at an off-axis location near the mirror edge. The beam is periodically reflected and refocused between these mirrors and then exits through the center of the input hole (defining the re-entrant condition) after a designated number of passes N, in a direction (slope) that is different from the entry slope. As a result, the total optical path traversed in the cell is approximately N×d. The pattern of reflected spots observed on the each mirror in these cells forms an ellipse. Re-entrant conditions for spherical mirror Herriott cells are restricted by certain predetermined ratios of the mirror separation d to the focal length f, and the location and slope of the input beam. For any re-entrant number of passes N, all allowed solutions are characterized by a single integer M. Thorough descriptions for the design, setup and use of these cells are given by Altmann (Altmann, J. R., et al., “Two-mirror multipass absorption cell,” Appl. Opt., vol. 20, No. 6, pp 995-999 (15 Mar. 1981)) and McManus (McManus, J. B., et al., “Narrow optical interference fringes for certain setup conditions in multipass absorption cells of the Herriott type,” Appl. Opt., vol. 29, No. 7, pp 898-900 (1 Mar. 1990)).
When the cell volume must be minimized relative to the optical path length or where a very long optical path (>50 m) is desired, it is useful to increase the density of passes per unit volume of cell. The conventional spherical mirror Herriott cell is limited by the number of spots one can fit along the path of the ellipse without the spot adjacent to the output hole being clipped by or exiting that hole at a pass number less than N. This approximately restricts the total number of passes to the circumference of the ellipse divided by the hole diameter, which in turn is limited by the laser beam diameter. For a 25-mm radius mirror with a relatively small 3-mm diameter input hole located 20 mm from the center of the mirror, a maximum of about (π×2×20)/3≈40 spots, or 80 passes is possible at best. Generally the hole is made larger to prevent any clipping of the laser input beam that might lead to undesirable interference fringes, and typical spherical Herriott cells employ less than 60 passes.
Herriott (Herriott, D. R. and Schulte, H. J., “Folded Optical Delay Lines,” Appl. Opt., vol. 4, No. 8, pp 883-889 (August 1965)) demonstrated that the use of astigmatic mirrors could greatly increase the spot density, and hence optical path length, in the cell. Each mirror has different finite focal lengths (fx and fy) along orthogonal x and y axes, and the mirrors are aligned with the same focal lengths parallel to one another. The resulting spots of each reflection on the mirrors create precessions of ellipses to form Lissajous patterns. Since these patterns are distributed about the entire face of each mirror, many more spots can be accommodated as compared to a cell with spherical mirrors. McManus (McManus, et al., “Astigmatic mirror multipass absorption cells for long-path-length spectroscopy,” Appl. Opt., vol. 34, No. 18, pp 3336-3348 (20 Jun. 1995)) outlines the theory and behavior of this astigmatic Herriott cell and shows that the density of passes can be increased by factors of three or more over spherical mirror cells. For these astigmatic mirror cells, light is injected through a hole in the center of the input mirror. Allowed solutions for re-entrant configurations are characterized by a pair of integer indices Mx and My, since there are now two focal lengths present along orthogonal axes.
Useful operation, however, is limited by severe design constraints. First of all, both Mx and My must simultaneously meet re-entrant conditions. For a desired N and variable distance d, the focal lengths fx and fy, must be specified to a tolerance of 1 part in 104. Since mirrors can rarely be manufactured to such tolerances, this cell as originally proposed is impractical for routine use. Kebabian (U.S. Pat. No. 5,291,265 (1994)) devised a method to make the astigmatic cell usable. However, his approach still remains difficult to achieve in practice and requires complex calculations and skill to get to the desired pattern. Furthermore, the astigmatic mirrors must still be custom made and cost many thousands of dollars for a single pair.
Recently, Silver (Silver, J. A., “Simple Dense Pattern Optical Multipass Cells,” Appl. Opt., vol. 34, No. 31, pp. 6545-6556 (1 Nov. 2005); U.S. patent application Ser. No. 10/896,608) invented a simpler, lower cost and more easily aligned dense pattern multiple pass cell using a pair of cylindrical mirrors. While the exact formulas for describing this cell are different from the astigmatic cell, both are characterized by the total number of passes N before re-entry and by integers Mx and My that characterize the number of half-rotations of the spot pattern (in polar co-ordinates) before exiting the cell.
The present invention operates by introducing a separate exit hole in the middle of the rear mirror, whereby the exit beam can be well separated from the entrance beam, and that this exit spot location is invariant to the cell configuration in terms of the designed number of passes or spot pattern. This added versatility permits the use of a wider variety of detectors and requires fewer optical components to collect the output optical beam.